Spent waste liquor, i.e. ‘black liquor’, produced in pulp manufacture is burnt in recovery boilers to recover the energy it contains as heat, on the one hand, and, on the other hand, to recover the chemicals it contains and to recycle them and thus to reduce the need for additional chemicals. Black liquor is fed into recovery boilers by black liquor guns, which convert continuous black liquor flow into droplet spray, which is sprayed into the furnace where it is burnt. A problem with known solutions is that the nozzle and joints of black liquor guns tend to burn. This is because, for instance, molten black liquor or burning coal accumulates on the black liquor gun as a result of flows occurring during combustion, which causes that the nozzle burns in course of time. In addition, oxygenous air flows through the mounting hole of the nozzle into the recovery boiler, which intensifies the burning of the material accumulated at the nozzle arm and causes that the nozzle arm becomes damaged more rapidly.
Significant factors during burning are poor cooling of the gun pipe and the above-mentioned accumulations on the outer and inner surfaces. As the nozzle of a black liquor gun may, in the worst case, become damaged and unusable even in one day and since one recovery boiler may include several black liquor guns, the repairing or replacement of the nozzles is an extensive and expensive service operation. At its worst, dozens of black liquor guns may burn and become damaged in one month. The burning and ensuing breaking of the black liquor gun cause significant extra costs, hinder the formation of an optimal droplet size and, in some cases, even cause danger, as black liquor, which is not in droplet format, flows directly to a char bed on the bottom of the recovery boiler.
Black liquor guns are serviced and their burning is prevented by different manual cleaning solutions, which remove material accumulated at the gun arms by means of different cleaning brushes and other devices. Some mechanical cleaning devices for black liquor guns have also been tested, but the results obtained were not satisfactory. Attempts have been made to clean black liquor guns and their mounting holes by blowing steam or air into the mounting hole, which has not considerably hindered the burning of the guns either.